Modern telephone switching systems are capable of providing a feature known as "conference calling" which allows a plurality of telephone subscribers to be joined in a conversation. Customarily, a conference circuit is employed to interconnect all subscribers of a conference call. To assure adequate transmission quality, the conference circuit incorporates signal amplification circuits which offset the effective transmission line losses occurring when several communication lines are interconnected. Furthermore, a mechanism must be provided to adjust the gain of the amplification device when lines are added to or disconnected from the conference circuit, since the effective transmission characteristics vary with the number of lines connected to the conference circuit.
Several schemes exist in the prior art which utilize conference circuits to provide multiple subscriber connections and which include signal amplification devices. One prior circuit disclosed in U.S. Pat. No. 3,423,538, W. B. Gaunt, utilizes a series of cascaded amplifiers arranged in a closed loop. One or more telephone communication lines is coupled to each amplifier and signals from each line are transmitted through the amplifier loop to all other lines. The Gaunt patent discloses several circuit embodiments each of which utilizes an arrangement comprising a plurality of amplifiers and negative impedance networks for maintaining adequate signal transmission quality. However, such an arrangement is uneconomical since it has high power consumption and necessitates extensive power circuitry. A further limitation of the arrangement is that it requires extensive external system control to assure that circuit oscillations do not occur when a subscriber line connected to the conference connection goes from an off-hook to an on-hook state.
A further prior art disclosure is U.S. Pat. No. 3,441,678, A. H. Budlong et al., involving a conference circuit arrangement having a plurality of individual conference circuits connected to a transformer-amplifier arrangement for coupling the circuits together. The Budlong arrangement requires extensive transformer and relay circuitry. Further, relay and supervisory functions require dedicated communication paths between a remote system control circuit and the individual conference circuits.